Temperature Scales
Simple Explanation
Temperature can be measured on several scales. Celsius (°C) sets 0° at the freezing point of water and 100° at its boiling point. Kelvin (K), the SI unit, starts at absolute zero (the coldest possible temperature) and uses the same size degree as Celsius: T(K) = T(°C) + 273. Fahrenheit (°F), used mainly in the United States, uses yet another zero point and degree size.
Why Do We Need It?
Kelvin is essential in physics because many formulas (like the gas laws) only work correctly with an absolute temperature scale that starts at true zero — using Celsius directly in those formulas gives wrong answers.
Formula
Temperature Scale Conversion
T(K) = T(°C) + 273; T(°F) = (9/5)T(°C) + 32
The Kelvin, Celsius, and Fahrenheit scales all measure the same physical temperature, just with different zero points and different-sized degrees — these formulas convert a reading on one scale to another.
- T(K)
- — temperature in kelvin (K) — the SI unit, starting at absolute zero
- T(°C)
- — temperature in degrees Celsius (°C)
- T(°F)
- — temperature in degrees Fahrenheit (°F)
When to use it: Whenever a temperature needs to be converted between the Celsius, Kelvin, or Fahrenheit scales.
Worked Example
Convert a temperature from Celsius to Kelvin
Convert 25°C to kelvin.
Why Does This Work?
Because a change of 1°C is defined to be exactly the same size as a change of 1 K, converting between the two scales is just a fixed shift (adding or subtracting 273) — no multiplication is needed, unlike converting to or from Fahrenheit.
Real-Life Example
Absolute zero
0 K (about −273°C) is called absolute zero — the coldest temperature theoretically possible, where particles have the minimum possible kinetic energy.
Because Kelvin starts at this true physical zero, it is the scale scientists use whenever a formula depends on temperature being an absolute (never negative) quantity.
Practice
Convert 100°C (the boiling point of water) to kelvin.
EasyConvert 20°C to degrees Fahrenheit.
MediumCommon mistake
Using a negative Kelvin temperature, or forgetting to add 273 before using temperature in a formula that requires an absolute scale — Kelvin temperatures are never negative in normal physical situations.
Quick Review
- T(K) = T(°C) + 273
- T(°F) = (9/5)T(°C) + 32
- Kelvin starts at absolute zero — the scale used in most physics formulas.